This invention relates to a starter having a planetary-type reduction gear housed therein, and more particularly to an improved starter in which an internally-toothed ring gear of the starter is made of a molded synthetic resin.
A conventional starter of this type is disclosed in Japanese Laid-Open Patent Application No. 58-120874, the structure of which is illustrated in FIG. 1 of the accompanying drawings. As shown therein, a starter 1 houses a planetary gear reduction mechanism 2 which has a sun gear 3 which is mounted on an output shaft connected to the rotor of an unillustrated direct current starter motor and a plurality of planet gears 4 which engage with the sun gear 3. The planet gears 4 are surrounded by and engage with an internally-toothed ring gear 5 which is press fit into a front bracket 6. The ring gear 5 is prevented from rotating by the engagement between radially outward projections 5a formed in the outer periphery of the ring gear 5 recesses 6a formed in the inner peripheral surface of the front bracket 6. The direct current starter motor and the ring gear 5 are secured to the front bracket 6 by unillustrated bolts which pass through holes 7 formed in the outer periphery of the ring gear 5.
The operation of this conventional apparatus will now be explained. When the unillustrated direct current starter motor is energized, the sun gear 3 is caused to rotate together with the rotor of the motor, and the planet gears 4 are caused to perform planetary motion about the sun gear 3. The speed of rotation of the planet gears 4 is less than that of the sun gear 3, and an unillustrated engine is started by the rotation of the planet gears 4. A reaction force which is applied to the ring gear 5 by the rotation of the planet gears 4 is transmitted to the front frame 6 by the engaging members, i.e., the projections 5a in the ring gear 5 and the recesses formed in the ring gear 6.
In this type of conventional apparatus, as the ring gear 5 directly engages the front bracket 6, high stresses develop in the ring gear 5 during starting, particularly when the engine dies during cranking and the inertia of the rotor of the starter motor produces a sudden increases in the torque applied to the ring gear 5. When the ring gear 5 is molded from a high polymer synthetic resin such as an engineering plastic, it can be damaged by the high stresses, and breakage can occur. In order to alleviate such problems, elastic buffering means are sometimes provided between the ring gear 5 and the front frame 6. However, the buffering means which are known in the art are complicated and do not adequately prevent deformation of the open end of the ring gear 5.
Furthermore, in a conventional apparatus such as the one illustrated in FIG. 1, when the ring gear 5 is molded, the provision of the projections 5a and holes 7 in the ring gear 5 can result in sink marks which produce deformation of the ring gear 5. This deformation causes the stresses arising in the ring gear 5 during use to be nonuniform, and locally high stresses can result in damage to the ring gear.